Small general-use overhead camshaft (OHC) engines in which an intake valve and an exhaust valve are driven to be opened and closed by driving a valve mechanism are known as valve-mechanism-equipped engines, an example of which is disclosed in Japanese Patent Application Laid-Open Publication No. 2006-152941. This valve-mechanism-equipped engine is provided with an intake rocker arm for driving the intake valve to open and close the valve, an exhaust rocker arm for driving the exhaust valve to open and close the valve, a cam member for driving the intake and exhaust rocker arms, and a transmission belt for transmitting the motive power of a crankshaft to the cam member.
In the valve-mechanism-equipped engine disclosed in Japanese Laid-open Patent Publication No. 2006-152941, the rotation of the crankshaft is transmitted to the cam member via the transmission belt to rotate the cam member, whereby the intake rocker arm and the exhaust rocker arm are made to swing. When the intake rocker arm and the exhaust rocker arm are made to swing, the intake valve and the exhaust valve can be opened and closed.
In a valve-mechanism-equipped engine, the shape of the combustion chamber, the location of the intake valve and exhaust valve, the location of the spark plug, and other factors are known to affect the combustion efficiency inside the combustion chamber.
However, since the valve-mechanism-equipped engine according to Japanese Laid-open Patent Publication No. 2006-152941 is a small general-use engine, there are selectivity limitations on, among other factors, the shape of the combustion chamber, the location of the intake valve and exhaust valve, and the location of the spark plug; and such limitations are a hindrance to increasing combustion efficiency.